Powertrain Controls - ECM/PCM

Code 55 Diagnostic Chart:

Fuel System Schematic Showing Gauges Connected For Testing:

CIRCUIT DESCRIPTION
The PCM's internal circuitry can identify if the vehicle fuel system is capable of supplying adequate amounts of fuel during heavy acceleration (power enrichment). When a power enrichment mode of operation is requested during "Closed Loop" operation (by heavy acceleration), the PCM will provide more fuel to the engine. Under these conditions, the PCM should detect a "rich" indication from the oxygen sensor. If this "rich" exhaust is not detected at this time, a DTC 55 will set. A plugged fuel filter or restricted fuel line can prevent adequate amounts of fuel from being supplied during power enrichment mode.

DTC 55 WILL SET WHEN
^ O2S voltage is less than 0.3 volt.
^ O2S is in "Closed Loop."
^ Engine is in power enrichment mode.
^ Above conditions met for 5 seconds.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. When an engine is idling or at steady cruise, the O2S voltage should vary from between approximately 100 mV to 900 mV. During power enrichment mode, more fuel is needed and O2S voltage should rise above 447 mV (0.447 volt.) This step checks to see if the O2S is operating properly.
2. Low fuel pressure or restricted fuel flow may cause a lean condition. For further diagnosis of the fuel system, refer to "A-CHARTS/CHART A-7 FUEL SYSTEM DIAGNOSIS." Related Tests, Information and Procedures

DIAGNOSTIC AIDS
^ If the fuel filter is restricted, the engine may receive adequate amounts of fuel at idle when the demand is low, but not under load (heavy acceleration, driving uphill, or cruising at higher speeds.)
^ Water or alcohol in fuel may cause low O2S voltage during acceleration.

Check also:
- Faulty or plugged injectors(s).
- Low fuel pressure.
- Low fuel level in tank.